Importance of the Invention and Prior Art
The electrical properties of dielectric materials that are of interest here include permittivity, measure of energy loss, direction of maximum permittivity, direction of maximum energy loss, and amounts of anisotropy both in permittivity and in energy loss. The dielectric sensor allows these electrical properties to be explored with different probing signals and used in the estimation of other properties of the material being measured. Although applications discussed in this disclosure are primarily to structural wood or wood products, the techniques are applicable to other dielectric materials.
In dielectric materials such as wood, there are a number of physical properties of interest. If the material is anisotropic, as is wood, these properties can be different along different directions, and the differences affect how the material can be used best. For example, in wood, bending modulus of elasticity (E) or stiffness and strength are functions of the direction of the wood fibers. When wood is used in structures, it is important that the structural properties are sufficient for the application. Strength is a structural property that cannot be measured nondestructively.
Present testing and sorting methods for wood are based on its visual appearance, nondestructive measurements of wood properties, or a combination of both. As well as being useful in estimating strength, measured properties can be important in their own right. For example, structural timber sold as machine stress rated lumber is measured and sorted for bending modulus of elasticity (E) in the longitudinal direction using technology described by Keller in U.S. Pat. No. 3,196,672. In this case E is used as an estimator for strength; but the E measure is itself useful in determining the rigidity of floor or other structural systems made of the tested product.
As another example of machine sorting of wood product, veneers used in laminated veneer lumber (LVL) are measured and sorted according to the propagation velocity of ultrasonic stress waves along a reference direction utilizing an invention by Logan in U.S. Pat. No. 4,201,093.
Industry acceptance of the Keller and Logan inventions in the form of production-line machines, the CLT Continuous Lumber Tester jointly produced by U.S. Natural Resources Inc. in Tigard, Oreg., and Metriguard Inc in Pullman, Wash., and the Model 2600 Veneer Grader produced by Metriguard Inc in Pullman, Wash., has demonstrated the value of machine sorting of wood product based on measurement or estimation of structural value. However, it is known that the present sorting methods for wood, although valuable, are still inadequate in accurately determining strength.
Wood can compete in strength with steel on a per unit weight basis. However, wood can be quite weak. The broad ranges of physical property values of wood and the present limited capability to sort based on these properties cause its structural design load values to be set relatively low for safety in its use. This means that the large bulk of wood which could be used safely at much higher loads is under-utilized and hence wasted.
Some aspects of wood which have been identified as contributing to its variable property values are the general direction of the wood fibers (grain angle), local deviation of grain angle such as occurs around knots, density, species, moisture content, orientation of growth rings with respect to machined wood surfaces and presence of juvenile or reaction wood.
The prior art by Norton et al in U.S. Pat. No. 3,805,156 and Bechtel et al in U.S. Pat. No. 4,972,154, describes how to measure grain angle in wood. Norton et al use a sensor with a rotating electric field, and Bechtel et al use a stationary sensor where one of the embodiments has two electric field patterns that are applied sequentially, first one and then the other during alternating time intervals. In either of these prior art technologies, the sensor has an operative surface in a sensor unit. During measurement, the operative surface is substantially parallel to a substantially plane (flat) surface of the material being measured.
In this disclosure, grain angle is the angle, relative to a reference direction, of the projection of the direction of an electrical property maximum onto a plane both parallel to a surface of the material being measured and including the reference direction. In the Norton et al and Bechtel et al prior art, the sensor unit outputs, from which grain angle is obtained, are determined by the direction of maximum permittivity.
Although grain angle in wood is important in the estimation of strength (Bechtel and Allen 1987), grain angle by itself does not provide an indication of the amount of alignment. The question raised is not just in what direction are wood fibers preferentially aligned, but also, how well are they aligned? Additional questions can be asked. Is some wood better aligned than other wood? If one piece of wood is better aligned than another, does it have greater strength? These questions can be asked both on a local scale and on a general scale.
Using a knot in wood as an example on a local scale, all the wood fibers about a knot are not aligned in the same direction; hence, a measure of the amount of alignment averaged over a sensed volume including the knot will not be as large as for a section of uniform straight-grained wood. A grain angle measurement using sensors of the type described in either U.S. Pat. Nos. 3,805,156 or 4,972,154 on a track along a piece of lumber going directly over a perfectly symmetrical knot will read zero grain angle at all points along the track. In this case, a measurement of grain angle alone misses the knot. The knot is missed because the sensed volume about points on the track before, over and past the knot have equal positive and negative contributions to grain angle, and these contributions average to zero just as they would if all the wood fibers were aligned in the zero direction. However, a measure of the amount of alignment is smaller in the vicinity of the knot than it is for the surrounding areas of uniform straight-grained wood and thus is useful in detecting and quantifying the knot.
As an example of reduced amount of alignment on a general scale, consider juvenile or reaction wood compared with normal mature wood. It is well known in the wood products industry that juvenile wood and reaction wood can have inferior structural value. According to Skaar 1988, speaking about longitudinal shrinkage: "This is believed to be the result of the difference between the fibril angles in the S2 layer in the cell walls of juvenile wood and those of mature wood" (Skaar, C., Wood-Water Relations, Springer-Verlag, New York, 1988). Skaar similarly describes reaction wood. We refer to the fibrils discussed by Skaar as microfibrils to distinguish them as constituents of wood fibers. From Skaar's description, these microfibrils spiral around the wood fibers with angle between microfibril direction and wood fiber direction being significantly greater for juvenile and reaction wood than for normal mature wood. Although the directions of the microfibrils, on the average, line up with the wood fiber direction, components of the microfibrils in directions transverse to the wood fiber direction are significantly greater for juvenile and reaction wood than for normal mature wood. Whereas a grain angle measurement measures zero grain angle so long as the juvenile or reaction wood fibers align with the zero reference direction, a measure of the amount of alignment can be less in this case than for normal mature wood because of the reduced longitudinal and increased transverse components of microfibril directions.
The increasing use of young trees has increased the amount of juvenile wood in the wood products industry. Development of equipment to detect and separate juvenile and reaction wood from demanding structural applications will have great value. Although the inventions of U.S. Pat. Nos. 3,805,156 and 4,972,154 are useful in measuring grain angle, those disclosures do not consider the problem of measuring the amount of alignment. The dielectric sensor of this invention, by providing a measure of the amount of anisotropy as an indicator for the amount of alignment, has promise for distinguishing juvenile and reaction wood from normal mature wood.
In some applications of wood, the orientation of the growth rings with respect to machined surfaces can be important. For example, the requirement of edge-grain wood is sometimes made for door framing, shingles and pencil slat material. The dielectric sensor of this invention has promise for detecting edge-grain versus non edge-grain material because of differences in electrical properties in the radial and tangential directions with respect to the growth rings in wood.
The present invention has application to reconstituted dielectric materials such as oriented flakeboard or fiberglass where the constituents (flakes or fibers) are formed together into a product having the fibers arranged preferentially in one direction. For process control or for sorting, it is desirable to have available a means for determining how well the material is aligned, that is, the amount of alignment. As will be seen, the dielectric sensor is useful in providing measures of the amount of alignment of these materials.
The amount of flake alignment in flakeboard is recognized as one of the most important variables in controlling the properties of flakeboard. There exists no other practical means of determining amount of flake alignment in the production line. Methods previously considered have included measuring velocities of stress wave propagation or bending E in orthogonal directions and optical analysis of individual surface flakes. These approaches have been useful in the laboratory for off-line quality control and for research, but they have not been proved in the production-line.
The density of wood is also a variable of interest. Other measured properties being equal, greater density usually implies greater strength both of the wood itself and at the interface between wood and fasteners such as nail plates. However, this is not always true. Knots, for example, are a common strength reducing characteristic, and knots have higher density than the surrounding wood. The dielectric sensor of this invention has value because it can distinguish between high density well-aligned material and high density poorly-aligned material such as around knots in wood.
Moisture content is a confounding influence in both the measurement of electrical properties and density of wood. Moisture content increases the density, the conductivity and the permittivity of wood. Skaar 1988 discusses the increasing permittivity with increasing density (dry wood basis) at constant moisture content. Further, for relatively small moisture contents (below fiber saturation), it can be inferred from James 1975 and corroborated by our experiments that the amount of anisotropy in permittivity increases with moisture content (James, W. L., "Dielectric Properties of Wood and Hardboard: Variation with Temperature, Frequency, Moisture Content, and Grain Orientation," USDA Forest Products Lab Report FPL 245, 1975). Thus, estimates of moisture content can be made with the dielectric sensor. Moisture content is an important variable in many applications of wood. Other sensors for measuring moisture content are available, e.g. from Wagner Electronics Products in Rogue River, Oreg., but these sensors are not useful in measuring amount of anisotropy or grain angle.
Other sensors have been used to measure permittivity of dielectric materials in a set of different directions. For example, James 1975, describes apparatus he used to measure the permittivity of and energy loss in wood for each of its three principal directions, along the wood fibers, transverse to the fibers in the radial direction of the growth rings, and transverse to the fibers in the tangential direction of the growth rings. The measurements by James have provided the wood products industry with useful data about the electrical properties of wood. However, the equipment he used was designed to provide accurate laboratory data and was not intended to be used in the production-line where the constraints of measurement time, specimen preparation and orientation of the specimens with respect to the apparatus are completely different.